VRK-1, a vaccinia-related kinase in C. elegans, has multiple functions throughout development. It is required for nuclear envelope formation during embryogenesis and for germ line proliferation and vulval morphogenesis during larval development. The Drosophila ortholog, NHK-1, is a histone kinase, mutant alleles of which affect chromosome morphology in oocytes. We have an ongoing interest in proteins that influence chromosome morphology and behavior during the meiotic divisions in C. elegans. We have been characterizing the phenotypes of embryos depleted of the vrk-1 gene, using RNAi. We have observed highly penetrant embryonic lethality. These embryos display defects in the meiotic divisions as well as a failure to decondense chromatin. As a result, these embryos arrest as multicellular embryos with very little DNA. This defect is not a result of nuclear envelope defects in the developing oocytes but does appear to affect the association of chromatin with the oocyte nuclear envelope. We are using real time imaging of live embryos expressing GFP-tagged histones to follow the behavior of the meiotic chromosomes during oocyte maturation in VRK-1-depleted oocytes and embryos. We are also using immunocytochemistry to determine whether any specific histone modifications or chromatin-associated factors are perturbed at this time to account for the observed meiotic defects. [unreadable] In the past year, we have also identified a mutant allele of vrk-1 from an embryonic lethal screen published 20 years ago (a deletion allele also exists and results in sterility). This new mutant displays many of the same phenotypes we have characterized by RNAi. This allele however now allows us to perform more directed genetic tests by asking whether mutations in other genes can enhance or suppress the vrk-1 mutant. This allele will also be useful in the near future for genetic suppressor screens to identify other factors that function in the VRK-1 pathway.